Information on mast cell tumors in pets

What is a Mast Cell?

A mast cell is type of white blood cell that plays a role in allergic response. Upon exposure to an allergen, mast cells degranulate, which refers to the release of chemicals and compounds including histamine.

Although histamine is commonly known for allergic reactions like sneezing, watery eyes and itchy skin, more serious effects can occur when it is released in large amounts due to degranulation. Serious, life-threatening allergic reactions are possible.

What Are Mast Cell Tumors in Dogs and Cats?

Mast cell tumors (MCT) are a common type of cancerous skin tumor in pets. They are found in both dogs and cats, most commonly in middle aged animals but can occur at any age. Most dogs with MCT only develop one tumor.

Most MCTs occur as nodules or masses in the skin and subcutaneous tissue of the body, limbs and head and neck. Mast cell disease can also affect the intra-abdominal organs. This type of visceral disease is considered more aggressive than when it presents in the skin tissue.

Feline Mast Cell Disease

There are a few differences between feline and canine disease. In cats, MCT make up 8-21% of skin tumors and are most commonly found in the head and neck. Siamese cats are the breed most affected. The disease is usually less aggressive in cats than in dogs, but it’s still important to contact your vet if you notice new lumps or masses on your cat so a treatment plan can be formulated.

Signs of Mast Cell Disease in Dogs and Cats

The signs and symptoms of mast cell tumors in pets depends on the disease’s grade and progression. Tumors can range from small, movable raised skin lumps to large, aggressive ulcerated areas of inflammation.
Common symptoms include:

• Raised lumps under the skin
• Vomiting
• Loss of appetite (anorexia)
• Blood in the stool
• Anemia (low red blood count)
• Abdominal pain

Tumors can grow and shrink in size over time, sometimes suddenly, depending on the degree of inflammatory response in the pet’s body due to the degranulation of the cells.

According to the American College of Veterinary Surgeons, this “occurs when granules inside the cells are released and irritate the adjacent tissues. These granules contain histamine, proteolytic enzymes (denature proteins), and vasodilatory substances responsible for the redness and swelling surrounding (the) tumor.”

What Causes Mast Cell Tumors?

Like any cancer, the cause of MCT is not completely understood by veterinary medicine. The culprit is likely a mix of risk factors, both genetic and environmental.
Any dog breed can get MCT, but the dog breeds most often affected are Boston terriers and Boxers, which make up about 50% of all cases. Bull terriers and Labrador retrievers are also particularly susceptible.
Genetic tests are available to test for a genetic mutation to a protein called KIT which is related to cell replication and division.

Diagnosing Mast Cell Tumors in Pets

A diagnosis is made with a needle biopsy, called a fine needle aspiration (FNA). This is a simple process that involves taking a small needle to collect a sample from the lump and placing them on a microscope slide. Mast cells will show up with dark staining granules, dark blue to purple granules that a veterinary pathologist can identify under a microscope.

If the pet’s tumor doesn’t have the dark staining granules, your vet may need to conduct further tests.

Once the tissue is removed, the grade and prognosis are determined via biopsy and histopathologic analysis.
Tumors can spread (called metastasis) to local lymph nodes and beyond to the bone marrow, blood vessels, visceral organs and skin in other parts of the body. Your vet will look for evidence of metastasis when your pet is diagnosed and may recommend additional tests such as abdominal ultrasound and chest x-rays.
MCT can resemble something as harmless as an insect bite, noncancerous lump, wart or other allergic reaction. Any skin abnormality should be reported to your veterinarian for evaluation, even if it appears benign.